Ultra-high-energy cosmic ray

Tiny bits fly through space. They move very fast. These bits have a lot of power.

One bit was very strong. It had the power of a baseball. That baseball was moving very fast.

These bits are very rare. Scientists only see a few. They use big tools to find them.

One tool is in Argentina. It is very large. It covers a lot of land.

We do not know where they start. They might come from far away. They might come from big black holes. They are a great mystery.

Tiny bits of matter fly through space. Most move fast. Some have much more power. We call these ultra-high-energy cosmic rays. These bits are very rare.

In 1991, a tool called Fly's Eye saw a huge one. It was called the Oh-My-God particle. This particle had a lot of power. It was like a baseball moving at half the speed of light.

Where do they come from? We do not know yet. They likely come from outside our galaxy. One idea is neutron stars. These are very dense stars. They can have strong magnetic fields. These fields might push bits of matter very fast.

Another idea involves active galactic nuclei. These are big black holes at the center of galaxies. These black holes might launch the bits into space.

Scientists use the Pierre Auger Observatory to study them. It is in Argentina. It is very big. It covers 3,000 square kilometers. This is about the size of Rhode Island. It uses water tanks and telescopes to find these rare bits.

Ultra-high-energy cosmic rays are tiny bits of matter from space. They carry an incredible amount of energy. This energy is much higher than most other cosmic rays. These particles are also extremely rare to find. Between 2004 and 2007, one observatory found only 27 of them. That is about one event every four weeks.

These particles work by carrying massive amounts of energy through space. One famous example is the Oh-My-God particle. It was seen in 1991 over Utah. This particle had the energy of a baseball moving at half the speed of light. Its energy was 40 million times higher than what we can make on Earth. Scientists use large tools to catch them as they hit our atmosphere.

People have been searching for these particles for a long time. John Linsley and Livio Scarsi made the first big discovery in 1962. They used an experiment in New Mexico called Volcano Ranch. Since then, many other teams have found even higher energy particles. The Fly's Eye experiment in Utah also made a huge discovery. Now, many different observatories around the world keep looking.

Scientists use special places to study these rare events. The Pierre Auger Observatory is in Mendoza Province, Argentina. It is a huge site covering 3,000 square kilometers. That is about the size of Rhode Island. It uses water tanks and telescopes to see the particles. Other projects include the Telescope Array and the HiRes detector.

We still do not know exactly where these rays come from. They likely come from outside our own galaxy. One idea is that they come from neutron stars. These are very dense stars with very strong magnetic fields. Another idea involves active galactic nuclei. These are huge black holes at the center of galaxies.

Ultra-high-energy cosmic rays, or UHECRs, are subatomic particles that travel through space with extreme energy. In the field of astroparticle physics, a particle is classified as a UHECR if its energy exceeds 1 EeV. This value is equal to 10^18 electronvolts, or about 0.16 joules. These particles carry far more energy than the rest mass or typical energies of standard cosmic rays. Because they possess such immense energy, they are incredibly rare. For example, the Pierre Auger Observatory detected only 27 such events between 2004 and 2007. This means scientists see roughly one event every four weeks in their surveyed area.

To understand how powerful these particles are, we can look at the "Oh-My-God particle." This particle was observed by the Fly's Eye experiment on October 15, 1991, over Utah. Scientists estimated its energy was approximately 50 joules. To visualize this, imagine an atomic nucleus with the same kinetic energy as a baseball traveling at half the speed of light. This single particle had 40 million times the energy of the highest-energy protons produced in any human-made particle accelerator. When such a particle hits Earth, it interacts with a proton or neutron in our atmosphere. The effective collision energy is calculated using the square root of double the product of the particle's energy and the mass energy of the proton. For the Oh-My-God particle, this resulted in a collision energy about 50 times greater than what the Large Hadron Collider can achieve.

The history of observing these particles began in 1962. John Linsley and Livio Scarsi made the first observation of a cosmic ray exceeding 16 joules. They used the Volcano Ranch experiment located in New Mexico. Since that discovery, many more high-energy events have been recorded. The Fly's Eye detector in Utah recorded at least fifteen similar high-energy events after its initial discovery. Today, many international projects work to track these particles. These include the Telescope Array, the HiRes detector, and the Yakutsk Extensive Air Shower Array.

Because UHECRs are so rare, scientists must build massive detection areas. The Pierre Auger Observatory in Mendoza Province, Argentina, is a primary example. It covers 3,000 square kilometers, which is roughly the size of Rhode Island. The observatory uses a cluster of water-Cherenkov tanks to detect particle shower components. It also employs four telescopes to watch the night sky. These telescopes look for the fluorescence of nitrogen molecules as shower particles pass through the air. This method helps scientists determine the direction of the original cosmic ray.

Scientists are still searching for the exact origin of these particles. Because their arrival directions do not correlate with the Galactic plane, researchers believe they are extragalactic. This means they likely originate from outside our own galaxy. One hypothesis involves neutron stars, which are extremely dense remnants of stars. In young neutron stars with spin periods under 10 milliseconds, magnetohydrodynamic (MHD) forces may accelerate iron nuclei. These stars, known as magnetars, have magnetic fields between 10^8 and 10^11 teslas. This is the strongest stable magnetic field observed in the universe.

Another leading theory focuses on active galactic nuclei, or AGN. These are supermassive black holes located at the centers of distant galaxies. Recent data from the Pierre Auger Observatory shows that UHECR arrival directions may correlate with these AGN. Some researchers suggest that the spin of a black hole could drive particle production. If ions are launched correctly, they might avoid losing energy to radiation deep within the galactic nucleus. Other possible sources include supernova remnants, relativistic supernovae, and gamma-ray bursts.

There are also complex theories regarding how these particles travel through space. As UHECRs move across cosmic distances, they interact with photons from the cosmic microwave background. This interaction creates a high-energy cutoff known as the Greisen–Zatsepin–Kuzmin limit, or the GZK limit. This limit helps explain the observed spectra of cosmic rays. Additionally, researchers like Yuri Pavlov and Andrey Grib have proposed a connection to dark matter. They suggest that dark matter particles near an AGN might decay or fall into a black hole via the Penrose process. This process could potentially create the high-energy protons we detect as UHECRs.